1. Field of the Invention
This invention relates to an improved process and apparatus for packaging yarn of the self-twist type.
2. Description of the Prior Art
In the manufacture of yarn, particularly yarn from synthetic fibers, there have been substantial developments in the area of false-twist and self-twist yarns because of various production advantages which can be realized using these techniques, and because such processes provide a shortened manufacturing route to a finished yarn product, and are therefore more economical as compared with conventional spinning and twisting processes.
As used herein, the term "false-twist" refers to a yarn in which a yarn strand is twisted to have alternate lengths of opposite direction twist with the sections joining the twisted sections having zero twist, which point will be referred to as a "node". The directions of twist are referred to as "S-twist" and "Z-twist", the appropriate letter being employed for twists in which the helices in twisted strands correspond with the middle portion of the appropriate letter.
The term "self-twist" is applied to yarns wherein two or more false twisted strands are brought together and permitted to ply therselves. The approximately equal torsional force of the same direction is stored in a pair or more of single yarns which are later brought into contact. Torque is released, permitting the single yarns to untwist, and in so doing, wrap around each other, forming a plied yarn. This plied yarn is especially suitable for carpet manufacture, because of the wide use of ply yarns in this industry for the carpet face yarn, and the economy of this manufacturing process.
However, self-twist yarns are extremely sensitive to tensional stress encountered in conventional winding operations, which are sufficient to cause gross changes in the yarn properties.
When a plied yarn is twisted, the singles yarns are forced into a helical path around one another, thereby shortening the length of the plied yarn strand. Tensional stress on plied yarn tends to straighten the helical path of the singles components, causing a torsional force to be developed in a direction counter to the singles helix in the ply strand. On conventionally processed yarns, the effect of this tension-induced helical torque is negligible, since the twist is uni-directional and the yarn cannot rotate to lose turns. In the case of a self-twisted yarn structure, however, a severe change in yarn twist occurs because sections of yarn on either side of a node have opposite helical torque when tensioned and the node rotates causing simulataneous loss of ply twist and increase of singles twist.
In order to use self-twist yarn, e.g., in a tufting machine creel, it must first be wound onto a cone or other suitable package. Tension on the threadline is essential to winding, as it contributes to proper package formation required to withdraw the yarn from the package in use, without sloughing or tangling. ("Sloughing" means that the take-off end pulls loosely wound adjacent wraps off the cone, resulting in a tangle.)
In carpet yarn systems, winding tensions may range from 100-300 gms. Lighter yarn counts for other end uses are wound at lower tensions, but it is also a fact that twist modification occurs at a lower level of tension on a finer yarn. This is because the helical torques discussed above are a function of yarn size (diameter of twisted member) so the twist-de-twist phenomena follow the same yarn size: twist: tension curves. In short, winding tensions cause a dramatic and deleterious change in ply/singles twist ratios in self-twist yarns.
This change in twist ratio tends to be made permanent in practice, since the yarn is placed on the cone in a tensioned and, therefore, elongated and de-twisted state. The cone may be stored for days or weeks prior to use, during which time the yarn is stress-relieved into its new and improper twist configuration.
This invention provides an apparatus and method for winding self-twist yarns into a reusable package, without loss or modification of the twist previously imparted to the yarn.